Comparison of aluminium vs copper power cables for use in

INSTALLATIONS
& CONTRACTING
The object of this report is to highlight both the positive and negative factors involved in
selecting aluminium cables for use in a project. Ever since the introduction of aluminiumcored electrical cable, cost savings have been the sought-after “pot of gold”. The price of
copper on the LSE skyrocketed, adding fuel to the fire.
Comparison of aluminium vs copper power
cables for use in industrial installations
Aluminium cables have been shunned
due to poor performance in the past.
This was mainly the result of lack of
knowledge. BHP Billiton has used
aluminium cables very successfully in
its giant aluminium smelters in Maputo
and Richards Bay. The main criteria for
the use of aluminium cable were the cost
savings as the vast majority of cables
installed were greater than 16 mm².
According to SANS 10142-1:2003 Para
6.3.1. “All conductors of nominal cross
sectional area less than 16 mm² shall
be of annealed copper”.
Para 6.3.2.1 “Conductors of nominal
cross sectional area exceeding 2,5 mm²
shall be stranded, except in the following
cases, where solid conductors may be
used” – (e) “Aluminium conductors of
nominal cross-sectional area of 16 mm²
or more”.
Size for size, the cost of aluminium cable
is considerably lower than the cost of
copper cable but there are many other
factors to be considered.
Aluminium cables come in either solid
or stranded configuration.
Stranded or solid
The cost of manufacturing stranded
cable is greater than solid so that solid
aluminium cable is used more frequently
than stranded. Large solid copper cable
would be almost impossible to bend and
is not allowed. Both stranded and solid
aluminium cables, larger than 16 mm²
may be used. Deep indent crimping
cannot be used on stranded cable so
by Robin Coombs, Bateman
Fig. 1: Solid aluminium cable.
hexagonal crimping must be utilised.
For the sake of this report stranded
aluminium cable is ignored.
Cable sizing
The main difference between aluminium
and copper cables is the current carrying
capacity and impedance. Aluminium
cable has a lower current carrying
capacity and greater impedance than the
equivalent copper conductor. Although
aluminium cable in sizes smaller than
25 mm² is available in other countries
and is used prolifically in India,
aluminium cable is only available from
size 25 mm² and up in South Africa.
When selecting the correct size of
aluminium cable the impedance must
be carefully evaluated.
July 2010 - Vector - Page 67
Fig. 2: Stranded copper cable.
Example 1
An example using a 35 mm² 3-core
steel wire armoured cable.
From Table 1 it can be seen that, size
for size, aluminium cable is smaller and
lighter than copper except for the current
rating which is very much lower. This
means that a larger cable size must be
selected if aluminium is to be used.
Sizing of a cable is achieved through a
process of selection based on certain
base criteria.
The following factors must be
considered.
l The size of the drive in kW
l Starting current multiples
l Power factor of the motor (generally
taken at 75% of full load)
Copper conductors
Aluminium
conductors
Current rating in air
135 A
93 A
Impedance
0,627 Ω/km
1,043 Ω/km
Cable diameter
30,5 mm
27 mm
Cable mass
2,08 kg/m
1,4 kg/m
Bending radius
305 mm
270 mm
Table 1: Comparison of cable characteristics
Type of starter (direct on line, variable
speed, soft start, etc.)
l Derating factors for altitude, ambient,
multiple cables on racks, solar
radiation
l Length of cable from the power
source (DB/MCC) to the equipment
(control panel, motor)
l Type of cable insulation
l Fault level of the system
For the purpose of this report only
balanced three phase electric motor
drives fed from a central motor control
centre are considered. Lighting and
small power is excluded. Cable derating
is ignored and only direct on line
starting is assumed. System fault level
is ignored.
l
Example 2
Motor: 90 kW
Line voltage: 550 V 50 Hz
Full load current (FLC): 120,81 A
Power factor of motor at 75% of full
load: 0,84 lagging
Starting factor: 7,6 times FLC
Cable insulation: PVC/SWA/PVC
Volt drop under running conditions not
to exceed 5% of nominal voltage
Volt drop under starting conditions not to
exceed 15% of nominal voltage.
Current ratings in air
Copper cable
Aluminium cable
35 mm² 3 core pvc/swa/pvc
135 A
93 A
50 mm² 3 core pvc/swa/pvc
-
110 A
70 mm² 3 core pvc/swa/pvc
-
145 A
Table 2: 70 mm² aluminium cable would be the smallest allowed.
Data calculated with a cable length of 100 m
Volt drop under
running conditions
Volt drop
under starting
conditions
Result
Cable size using copper
conductors 35 mm², 3 core
PVC/SWA/PVC/PVC/Cu
2,15%
7,42%
Cable suitable
Cable size using aluminium
conductors:
70 mm², 3 core
PVC/SWA/PVC/PVC/Al.
2,37%
5,14%
Cable suitable
Table 3: Example 2.
Data calculated with a cable length of 100 m
Volt drop under running
conditions
Volt drop
under starting
conditions
Result
Cable size using copper
conductors: 35 mm², 3 core,
PVC/SWA/PVC/PVC/Cu.
4,30%
14,85%
Cable
suitable
Cable size using aluminium
conductors:
70 mm², 3 core,
PVC/SWA/PVC/PVC/Al
2,74%
10,28%
Cable
suitable
Table 4: Example 2.
July 2010 - Vector - Page 68
From Tables 3 and 4 it can be seen that
by using aluminium cable, the selected
cable will be two sizes larger.
Using our example if a copper cable
is used, a 35 mm² is suitable whereas
using aluminium cable, a 70 mm² is
required.
Terminations
When terminating copper conductors tin
plated copper lugs are crimped on to the
conductor with a hexagonal crimping
tool. This results in a solid copper to
copper joint that can be bolted onto the
terminating copper connection.
When terminating aluminium
conductors a different approach is
required. The lug must be a special
Al-Cu type such that the aluminium
cable core is crimped into the
aluminium tube which is bonded to
the (bi metallic) copper spade. This
can now be safely connected to the
copper connection.
The crimping tool for this type of lug
is not hexagonal but a circular deep
indent, two of which are required. This
indent pierces through the outer layer
of aluminium thus passing through
the aluminium oxide layer of the outer
skin. This applies to solid aluminium
conductors. A hexagonal crimp can be
used provided that certain precautionary
steps are taken. The outer skin of
aluminium cores develops an aluminium
oxide layer that reduces electrical
conductivity.
To allow the use of hexagonal
crimping this layer must be completely
removed and the surface “roughed up”
immediately prior to crimping. For this
reason hexagonal crimping of solid
conductors is not recommended.
If an aluminium conductor is crimped
into a copper lug, two things happen.
l An electrolytic reaction takes place
when moisture is introduced, causing
corrosion.
l The temperature coefficient between
aluminium and copper is such that
when heated, under load, the two
metals expand and contract at a
different rate. (17 x 10-6 for copper
and 23 x 10-6 for aluminium) This
causes looseness within the crimp
with subsequent overheating and
finally failure. This can also be the
cause of fires.
Size of lugs
There is a large difference between the
size of copper and bi-metallic lugs.
Using our example a standard 35 mm²
copper lug is 37 mm long whereas a
ALU300 R21 - P36 T2
Finnish
AI conductor Type no for
punch and matrix temperaturein mm2
class
Type no for when cripming
punch and matrix
when performing
16
Stud size
Fig. 3: All indent crimping sequence.
Cu 240 - 30
70 mm² bi-metallic lug is 86 mm
long.
Thus it can be seen that, in most
instances, more space is required
for terminations. This can result in
the terminating box being too small,
requiring the addition of a larger box.
In many instances this is not physically
possible so an additional interconnection
box needs to be introduced where the
aluminium cable is terminated on to
short bars and the smaller copper
cable connecting to the motor. It would
be preferable to install local isolators
where this aluminium/copper transition
can take place.
Cu-conductor Type no for
hexagonal die
in mm2
ALU 300 - R21 - P36
T2
See above
Fig. 4: Termination lugs and ferrules.
Availability (South Africa)
Both copper and aluminium cable have
the same delivery periods although
aluminium, as it is not as popular,
cannot readily be sourced in a hurry. It
is manufactured on demand whereas
copper cable is kept in stock.
The order quantities are the same as
are the drum lengths. Copper lugs
are manufactured in SA so are readily
available. Bi-metallic lugs are imported
but the local stockist carries a fairly large
consignment. Availability in remote
locations could be a real problem.
Vibration and corrosion
Aluminium cable is prone to cracking
and failure when subjected to vibration.
In nearly all electrical installations
vibration is present. Motors and
transformers produce vibration so the
use of aluminium cable will be more
susceptible to cracking than copper
(quote from the Electrical Contractors
Association). There are recorded cases
of aluminium overhead lines failing due
to Aeolian (wind generated) vibration.
Vibration damage is even more serious
in solid aluminium conductors.
Aluminium has excellent corrosion
resistance but this is only true for aircraft
grade aluminium and coated aluminium
as used in window frames etc. Aluminium
used for cables corrodes when installed
in damp conditions so is not really
suitable for damp process plants and
Fig. 5: Comparative dimensions.
is not recommended (quote from the
Electrical Contractors Association).
Contrary to the above, aluminium cables
have been used successfully on the
Mozal and Hillside aluminium smelters,
both at high humidity coastal locations,
where no vibration cracking or corrosion
have been experienced. The only failures
experienced are where inadequate
measures were taken in the selection of
lugs or masking with corrosion-inhibiting
Denso putty or tape. All motors were
fitted with local isolators where the
transition from aluminium to copper
cable took place and no vibration was
transmitted to the aluminium cable
from the motor. The 360 kA DC open
aluminium bus bar system displayed no
symptoms of corrosion and no special
precautions were taken to protect it from
the elements.
Installation
The qualification of the installation
electrician is the same for both copper
and aluminium. Aluminium is more
difficult to work with so the cost of
termination is higher. On average
the additional cost is in the region of
July 2010 - Vector - Page 70
12 to 15% of the cost of termination.
The actual installation of the cable on
to racks costs the same.
Cable support
Aluminium cable is smaller and lighter
than copper but due to the increase in
size a slightly larger ladder support may
be required in some instances.
If, as in our example, there were
10 drives following the same route
then the size of ladder required for
copper cables would be 305 mm wide
whereas for aluminium cable it would be
330 mm wide. A 400 mm wide rack
would be chosen.
The mass of copper cable on that
rack would be 21 kg/m whereas the
aluminium would be 20 kg/m, thus not
much different.
Costing
Aluminium cables are cheaper than
copper. Using our example we get the
following:
35 mm² 3 core PVC/PVC/SWA/PVC/Cu
cable – R 140,36/m (2007)
Fig. 6: 360 000 A DC positive aluminium busbar installation.
Cost of Cu cables greater than 16 mm²
R5 203 735
27% saving on above
R1405 008
Additional cost for junction boxes at motors
R425 000
Additional cost of cable from junction box to motor
R169 556
Mounting of junction boxes
R34 000
Cable cost savings
R776 452
Table 5: LV cables.
A typical South African process plant
project (2007)
All cables of 25 mm² and greater from the
cable schedule were used to evaluate the
possible savings by using aluminium cable
in place of copper. The following costs were
extrapolated using the estimated saving
percentage from this report.
70mm² 3 core PVC/PVC/SWA/PVC/Al
cable – R 98,55/m (2007)
Thus for an installed length of 200 m
l Using 35 mm² copper cable costs
R28 072
l Using 70 mm² aluminium cable
costs R19 710
This shows a significant saving but
the additional cost of lugs and labour
must be added. A hidden cost is the
requirement of larger enclosures/
terminal boxes that is rather difficult to
quantify.
l Cost of 35 mm² copper lugs R10,75
ea x 6 = R64,50
l Cost of 70 mm² bi metallic lugs
R68,35 ea x 6= R410
Cost of cable
R1 492 815
Estimated 18% savings
R268 706
Total savings for the use
of Al cable
R1 045 158
Table 6: HV cables.
The termination labour cost of 35 mm²
copper 3-core cable is R151,67 thus
terminating both ends costs R303,34.
Termination labour of a 70 mm²
aluminium 3 core cable is R225,72 thus
terminating both ends costs R451,40.
Total installed cost for copper cable is
R28 166,84.
Total installed cost for aluminium cable
is R20 571,40.
Saving by using aluminium cable is
R7595,44 per drive of 90 kW (±27%).
Summary
The use of aluminium cable is significantly
cheaper than copper so when used
on a project, the cable costs will be
considerably less and, depending on the
July 2010 - Vector - Page 71
amount of large cables involved, could
present considerable cost savings. The
cost of lugs is, conversely, much greater
than copper.
Added to this is the additional cost of
labour as aluminium cable is more
difficult to work with. The cost of tooling
is also higher but, obviously, not to the
same extent.
The biggest drawback to the use of
aluminium cables is the risk factor.
l Once an aluminium cable has been
installed it cannot safely be moved.
Further, when handling aluminium
cable great care must be exercised
to ensure that it is not bent too many
times as this leads to cracking.
l The bending radius of aluminium
cable is smaller than that of copper
but, once bent, it cannot safely be
straightened.
l Great care must be taken in applying
moisture inhibiting putty or grease
on all joints to prevent electrolytic
reaction.
l Corrosion can be a serious risk
factor in damp conditions.
Providing that the associated risk is
acceptable and the correct installation
methods are strictly applied then the
use of aluminium cable can result in
approximately 27% savings for the
supply and installation of cables.
Contact Robin Coombs,
Bateman NV (retired),
Tel 011 849-1527,
rcoombs@iburst.co.za